1. Field of the Invention
The present invention relates in general to an internal combustion engine, and in particular to a fuel injection system of an internal combustion engine.
2. Description of Related Art
Outboard motors are increasingly being provided with fuel injection systems. Engines that are equipped with fuel injection systems offer such advantages as improved fuel economy, lower emissions, increased engine power, and better fuel distribution into each engine cylinder.
A fuel injection system, as referred to in a general sense, includes an induction system and a fuel supply system that supplies fuel to fuel injectors of the system. The induction system often includes an intake silencer. A plenum chamber of the intake silencer acts as a source of air for the engine. The engine draws air from the plenum chamber through a plurality of throttle valve and into separate intake passages, where the fuel injectors inject fuel into the air stream.
Many outboard motors now employ such a fuel injection system with a two-cycle, crankcase compression engine. In a two-cycle engine, each separate intake passage of the induction system leads to a dedicated crankcase chamber. The fuel-air charge, which is formed by the induction system, is delivered to the crankcase chamber where a piston compresses the charge before delivery to an associated combustion chamber. Although a check valve usually operates between the corresponding intake passage and the crankcase chamber, the position of the associated piston influences the pressure within the respective intake passage. The pressure within each intake passage varies as the associated piston moves through its cycle.
Pressure variations within the intake passage detrimentally affect the amount the fuel injectors inject into the corresponding intake passage. The amount of fuel injected by the fuel injector depends on the differential between the pressure within the intake passage and the fuel pressure within the injector, which a pressure regulator of the fuel supply system commonly establishes. Pressure fluctuations within the intake passage consequently cause the pressure differential between the fuel pressure and the air pressure to vary. The resulting fuel/air ratio of the charge delivered to the associated combustion chamber thus may vary between engine cycles.
Inconsistency of the fuel/air ratio in the formed charges also can vary between the engine's cylinders. At any given time during engine revolution, the positions of the pistons in a multi-cylinder engine are different. For instance, in a three cylinder engine, one cylinder may be at top-dead-center, a second cylinder may be moving toward top-dead-center and a third cylinder may be moving toward bottom-dead-center. As a result, at any given time, the pressures within the associated intake passages differ from one another. Such variations in some prior engines have lead to inconsistent fuel injection volume between the cylinders, causing the engine to run rough.
Inconsistency of the fuel/air ratio of the charges delivered to each cylinder also occur in some prior engine because separated fuel vapors are introduced into only one of the intake passages. In such engines, the fuel supply system includes a vapor separator that separates gaseous fuel from liquid fuel. The vapor separator usually supplies the gaseous fuel to only one of the intake passages. As a result, the intake passage that receives the gaseous fuel operates at a richer fuel/air ratio than the other intake passages.
In addition, fuel blown back into the plenum chamber also is commonly introduced into only one of the intake passages. In prior two-cycle engines, the throttle valve and reed valve associated with each cylinder lie near each other in order to enhance the responsiveness of the engine. This arrangement, however, often results in a portion of the injected fuel being blown back into the plenum chamber as the pressure within the associated intake passage fluctuates. Under some conditions during the engine cycle, the pressure differential between the throttle passage and the plenum chamber produces an air flow toward the plenum chamber. The blown back fuel tends to collect and vaporize at the bottom of the plenum chamber. As a result, the air-fuel ratio in the lowermost intake passage becomes excessively rich.